Method and apparatus of coupling light into a light guide using error detecting and correcting systems. A rectangular light guide is mated with an X-axis optical fiber and with a Y-axis optical fiber. An X-axis photodetector senses light that couples from the light guide into the X-axis optical fiber, while a Y-axis photodetector senses light that couples from the light guide into the Y-axis optical fiber. An error detector compares the outputs of the X-axis and the Y-axis photodetectors to produces error signals that depend on the X-axis, the Y-axis, and the Z-axis positional errors. The errors signals are applied to an error correcting system that adjusts the relative position of the light guide and the light directed onto the light guide.